1. Field of the Invention
The field of the present invention relates to communications and, more particularly, to a technique for cyclic time hopping in a time division multiple access communication system.
2. Description of the Related Art
A mobile telephone system may generally comprise a set of "user stations", typically mobile and the endpoints of a communication path, and a set of "base stations", typically stationary and the intermediaries by which a communication path may be established or maintained.
One method of communicating with multiple user stations is time division multiple access (TDMA), wherein a time frame is divided into a plurality of smaller time units, or time slots, and communications from the base station and from the user stations are separated in time so as to avoid collisions. Transmissions may also be distinguished by using different assigned frequencies, thereby performing frequency division multiple access (FDMA), or by using different assigned codes, thereby performing code division multiple access (CDMA).
In a typical TDMA system, a user station is assigned a time slot for communicating with a base station. A single base station thereby communicates with multiple user stations, one in each time slot. The communication may be duplex where, for example, a forward link transmission and a reverse link transmission occur within a given time slot. Or, a forward link transmission may occur in an assigned time slot of a first time frame, and a reverse link transmission in the same relative time slot of a subsequent time frame.
A problem that has arisen in some TDMA systems is that the regular, periodic burst pattern of time division signals, particularly those associated with user stations, can cause undesirable side effects. In particular, the frequency at which the TDMA bursts occur from a particular source may lead to unwanted spectral components. Consequently, the TDMA transmissions can interfere with sensitive electronic equipment, and can, for example, couple into hearing aids to produce audible results, or may interfere with the operation of heart pacemakers. According to one exemplary study, as many as 16% of hearing aids experience interference when located near a handset of a GSM system (a type of TDMA system), and as many of 84% of hearing aids experience interference when placed directly next to a GSM handset. These types of undesired side effects may hinder deployment of a TDMA system where interference with sensitive electronic equipment is anticipated.
Further, radiation emissions from transmitters may be limited by government regulations or other similar restrictions. For example, regulations and standards may be promulgated by the American National Standards Institute (ANSI). One example is the ANSI C95.1 standard, which defines the allowable specific absorption rate (SAR) for periodic low frequency pulsed transmissions such as may be emitted, e.g., by cellular telephones. To increase the likelihood that TDMA and similar systems will be able to comply with governmental and other regulations, it is generally desirable to reduce potential interference from transmission sources as much as possible, without substantially impacting system performance.
Accordingly, it would be advantageous to provide a TDMA communication system that minimizes interference caused by low frequency pulsed transmissions arising from a periodic burst pattern, particularly with respect to sensitive electronic equipment such as hearing aids and pacemakers. It would further by advantageous to have such a system that minimizes or reduces radiation emissions for transmitters governed by various promulgated standards, including, for example, the ANSI C95.1 standard governing low frequency pulsed transmissions.